The present invention relates to a thermal protection device for use in an electric circuit of an electrical apparatus, in particular an expendable thermal protection device for use in electric motors in the field of motor vehicles.
DE 30 45 540 A1 discloses a protection device for an electric circuit, in particular in an electric motor, e.g. for a radiator fan or an air conditioning system.
For many electric motors used in the field of motor vehicles, a thermal protection device, which is also referred to in the following as a thermal protector for the sake of simplicity, is used on the basis of what are known as soldered chokes. Said chokes consist substantially of a coil spring consisting of wire material, i.e. one or more wire portions, the wire material breaking at a point which is generally in the centre of the coil spring based on the longitudinal orientation thereof, and the ends of the wire portions, which are also generally referred to in this application as contact means connected in series, are soldered using a solder material while being biased against one another.
Said chokes are arranged in an electric circuit of an electric motor and have the function of both eliminating interference in the electric circuit and protecting the electronic components against undesired temperature increases. If a determined limit temperature is reached, the solder material which is electrically interconnecting the two wire-portion ends melts. Owing to the mechanical bias prevailing between the two wire-portion ends, the two soldered wire-portion ends spring apart and the electric circuit is broken. Since it is impossible to reverse the breaking, this is also referred to as an expendable thermal protector.
Conventionally, such soldered chokes contain lead-containing solder material, generally Sn63Pb, having a melting point of approximately 183° C. However, there have been Europe-wide guidelines in force for a few years, according to which, inter alia, lead-containing solder materials can no longer be used for consumer products. Lead-free solder materials having correspondingly higher melting points therefore have to be used. The melting point or melting range of lead-free solder material is typically in the range greater than 220° C.
Electric motors conventionally comprise what is referred to as a holding device, which is also termed a brush holder. This holding device is used to receive, hold and guide power supply apparatuses, such as carbon brushes. The holding device can, for example, be a unit consisting of a voltage regulator and a pair of carbon brushes, which are used to supply the electric motor with power, for the operation thereof, by means of sliding contacts of a commutator arranged on the rotary shaft of the electric motor. The holding device is generally produced from a plastics material, the soldered choke being directly fixed to the brush holder.
If the material of the holding device or the brush holder material is not suitable for use at higher temperatures, the relatively high melting point required for the lead-free solders occasionally leads to failure of a fire protection system if additional measures are not taken. Since the material of the holding device softens, the carbon brushes can no longer be adequately pressed against the commutator in order to ensure a reliable electrical contact. There may therefore be a time delay in a temperature increase acting on the soldered choke, and therefore said choke is not triggered in a timely manner, i.e. the circuit is not broken in a timely manner.